1. Field of the Invention
The present invention relates to a method for treating a contact lens to impart a hydrophilic nature thereto. More particularly, it relates to a method for treating the surface of a contact lens having a strong hydrophobic nature, such as a hard contact lens or a water-nonabsorptive soft contact lens to impart a hydrophilic nature thereto advantageously.
2. Discussion of Background
Contact lenses which are-practically used at present, may be classified generally into those made of water-absorptive material and those made of water-nonabsorptive material. As water-absorptive contact lenses, those made essentially of a polyhydroxy methacrylate or polyvinyl pyrrolidone may be mentioned. As water-nonabsorptive contact lenses, those made essentially of polymethyl methacrylate or silicone rubber, or those made of a copolymer of a polysiloxanyl methacrylate with methyl methacrylate or the like, having oxygen permeability, may be mentioned. Water-absorptive contact lenses have a merit that they give a comfortable feeling to the wearers, but they have a demerit that they are likely to be contaminated with bacteria, and even when the cornea is damaged, irritation is little, whereby there is a danger of leading to a serious disease without notice. On the other hand, water-nonabsorptive contact lenses are relatively free from contamination with bacteria, but their surface has water repellency. Among them, contact lenses having high oxygen permeability have a particularly strong hydrophobic nature.
On the other hand, vital tissue-compatible contact lenses made of a vital component such as collagen, have been proposed. Collagen is a fibrous protein constituting from about 20 to about 30% of the total protein of the whole body of a vertebrate and serves mainly as support tissues and cell skeletons. Such collagen has very weak antigenicity and is composed of hydrophilic amino acids, and it is provided with suitable conditions as a lens material, such as transparency. However, when it is to be molded into lenses, it has problems with respect to the physical strength, susceptivity to contamination with bacteria and difficulty in processing.
On the other hand, various proposals have been made for a method of imparting a hydrophilic nature by modifying the surface of the above-mentioned water-nonabsorptive contact lens having a strong hydrophobic nature. A contact lens poor in the hydrophilic nature of the surface or poor in the wettability with tear, is likely to have a problem in the vision-correction when it is put on the eye, and the movement of the lens on the cornea is poor, thus leading to a problem in fitting or a problem of giving an uncomfortable feeling to the wearer. Further, when such a contact lens is put on the eye, a lipid from sticky secretion collecting in the eye tends to adhere and form a stain on the lens surface, thus leading to fogging of the lens and impairment of the visibility.
As a method to impart a hydrophilic nature to the surface of such a water-nonabsorptive contact lens having a strong hydrophobic nature, it is conceivable to employ a plasma treatment method or a chemical treatment method using an acid or an alkali. However, such a method has a problem in the durability of the hydrophilic nature, and if the treatment to impart hydrophilicity is to be applied again, the corresponding work and an extensive apparatus will be required.
Further, Japanese Unexamined Patent Publication No. 271410/1988 proposes a method for imparting a hydrophilic nature to a contact lens, which comprises contacting a contact lens to a solution containing a hydrophilic monomer and a photosensitizer selected from the group consisting of aromatic ketones and quinones, followed by irradiation with ultraviolet rays. However, this method has a problem that the hydrophilic monomer and ultraviolet rays are not safe to a vital body, and the contact lens itself is likely to undergo a deterioration such as coloring or deformation by ultraviolet rays.
Further, U.S. Pat. No. 4,801,475 proposes a method for imparting a hydrophilic nature, which comprises coating an aqueous mucopolysaccharide solution on a plastic such as a contact lens, dehydrating it with a solvent missible with water, and then crosslinking and fixing it with a polyisocyanate, followed by drying. However, such a method is not only cumbersome but also has a problem that it is rather difficult to uniformly coat the mucopolysaccharide on a contact lens, and the solvent used for dehydration tends to give an adverse effect to the lens standards.
Whereas, as a method for conducting the treatment for hydrophilicity relatively simply, a method for conducting the surface modification of a contact lens has been proposed which comprises dipping a contact lens in a solution containing a hydrophilic polymer to deposit the polymer on the surface of the contact lens.
As such a solution containing a hydrophilic polymer, for example, Japanese Examined Patent Publication No. 37910/1973 discloses a solution for contact lenses, which contains a water-soluble polymer such as polyvinyl alcohol, hydroxyethyl cellulose or polyvinyl pyrrolidone. However, such a treating solution has a drawback that the adsorptive force of the polymer to the contact lens surface is weak, and there is no adequate effect to a material having a strong hydrophobic nature, such as a material of a water-nonabsorptive contact lens containing a substantial amount of a silicon component or a fluorine component.
Further, Japanese Unexamined Patent Publication No. 246718/1988 discloses a lens-treatment solution for treating the surface of a contact lens, which comprises a polymer material having an ionic charge. With such a solution, however, it is required that the charge density of the contact lens has a certain level, and accordingly, it is necessary to conduct a treatment to increase the charge density of the contact lens surface prior to the treatment to impart the hydrophilic nature. Thus, the process tends to be cumbersome. Besides, even when a contact lens is treated with-such a solution of a polymer having ionic groups, the durability is very poor, and there has been a drawback that the water wettability tends to be poor when the contact lens treated with such a solution is touched by a finger for application or when blinking only a few times after such a lens was put on the eye.
Further, U.S. Pat. No. 4,786,436 proposes a hydrophilic solution for a contact lens utilizing the hydrophilic nature of collagen and the characteristic of the compatibility of collagen with a vital body, and Japanese Unexamined Patent Publication No. 50014/1989 proposes a solution for a contact lens, which contains a cationic polymer such as a cationic derivative of polysaccharide, such as Chitosan. It is possible to impart a hydrophilic nature to a hydrophobic contact lens surface by dipping the contact lenses to such solutions. However, in each case, there has been a drawback that the durability of the hydrophilic state is very poor, and the water wettability tends to be poor when the contact lens treated with such a solution is touched with a finger for application or when blinking only a few times after it was put on the eye.